The present invention relates to electrical interlock switches, and particularly to a magnetically actuated interlock switch which will produce an open circuit in the event of failure of one of its components.
Interlocks are switching devices used to protect machinery and operators by ensuring that electrical power is available to certain parts of the machinery only when certain components are located in required positions with respect to each other. For example, to ensure that separately driven parts of machinery do not collide with each other, an interlock provides electrical power to drive a first part of the machine only when a second part is safely out of the path of the first. Interlocks are often used to permit electrical equipment to operate only when a shield, cover, or other safety appliance is properly located in its operative position. Of at least equal importance is the use of interlocks to be sure that a machine operator is safely out of the way before power is made available to drive dangerous parts of machinery such as cutting, grinding, forging or punching machines.
In the past, interlocks were primarily mechanical switches and required frequent adjustment and maintenance in order to operate properly. Mechanically operated interlock switches often had less than the desired reliability, and it was easily possible for many such mechanically operated interlocks to be overridden in the interest of faster operation of machines equipped with mechanically operated safety interlocks. More recently, because of their greater reliability, durability, and resistance to being bypassed, magnetically actuated reed contacts have been used in interlocks where the current to be controlled is not too great, and solid-state switches such as triacs have been used for applications having somewhat higher loads. However, in the event of overloading, or as a result of long use, magnetic reed contacts may stick in a closed state, particularly if subjected to overload in a closed state. Triacs may also fail in a conductive state if subjected to electrical overload, or may be induced by spurious voltages to switch to a conductive state inappropriately.
While an interlock may be connected in series with a fuse, the fuse can protect only against overloads and does not provide the function of the interlock as regards the machinery or machine operators, if the interlock simply remains in a conductive state when it should have opened a power circuit. As a result, if a mechanical contact, a magnetic reed contact, or a triac of a prior art interlock fails in a conductive mode, when the interlock is supposed to have opened a power supply circuit, power will still be available and may permit operation of a portion of a machine at the wrong time and thus result in damage to the machine or injury to an operator.
What is needed, then, is an interlock device which is long-lasting, does not need frequent adjustment or maintenance, cannot be easily overridden or bypassed, and which reliably results in power being definitely made unavailable to the machine or equipment protected by the interlock, in the event of failure of a component of the interlock.